In many facilities with continuous digesters for producing kraft pulp, the mills have reached their production capacity yet would like to increase their pulp production without the necessity for building entire new lines. Other mills would like to decrease the demand in their bleach plants. Other mills wish to increase the strength of softwood pulp so that the proportion of softwood pulp in paper making, or other applications, can be increased. All of these objectives can be achieved according to the present invention by providing extended kraft delignification using a pressure diffuser.
According to the present invention, a pressure diffuser is operatively connected to the pulp discharged from a continuous digester. No, or very little, washing is done in the bottom of the digester, therefore the entire digester can be devoted to cooking, and thus production can be increased. Any wash liquid that is supplied to the bottom of the digester is throttled or heated so that the pulp discharged from the digester is at high temperature, typically having a blow temperature over about 300.degree. F., if extended delignification is to be immediately practiced thereafter.
By providing a particular treatment liquid and by taking advantage of the efficient diffusion that is possible using a pressure diffusion washer, it is possible to effect further delignification in the pressure diffuser. A pressure diffuser is a commercially available piece of equipment, sold by Kamyr, Inc. of Glens Falls, N.Y. and Kamyr AB of Karlstad, Sweden. Such a diffuser is illustrated and described in Canadian patent 1,173,603, and in an article entitled "Pressure Diffuser--A New Versatile Pulp Washer" by Knutson et al, SPCI-84 World Pulp and Paper Week Proc., Apr. 10-13, 1984, pp. 97-99.
In the first stage of a pressure diffuser, delignifying liquor is added to the pulp. The purpose in adding the liquor is to replace the water around the pulp fibers with the delignifying liquor. This is effectively accomplished by supplying the liquor to the pulp at a liquor-to-water (in the pulp) ratio of about 0.2-1.3.
The delignifying liquor must have a low concentration of dissolved lignin (i.e. a low dissolved solids content). Typically the water in the pulp has a dissolved solids concentration of about 12 to 16%. The delignifying liquor must have a concentration less than 12%, and preferably about half (e.g. 6%) or less of the pulp water concentration, and optimally 4% or less. The alkali concentration must be at least 2 gm/l effective alkali (Na.sub.2 O), and preferably is at least about 8 gm/l, and about 20-55 gm/l is particularly effective. The delignifying liquor also must be at high temperature. The exact temperature will depend upon the blow temperature of the pulp (or other pulp temperature where extended delignification is not practiced immediately after continuous digesting). The higher the temperature of the pulp actually encountered by the delignifying liquor, the lower its temperature need be. Typically, however, the temperature of the delignifying liquor must be at least about 300.degree. F., and for a blow temperature of about 320.degree. F. would optimally be around 315.degree. F.
By replacing the water around the pulp fibers with delignifying liquor, the K-number of the pulp is significantly reduced. In actual working examples of the invention, the K-number has been reduced, on some occasions, over 6, a very significant reduction.
Since mild steel, typical of some equipment that is in pressure diffusers, corrodes when subjected to alkaline solutions at high temperatures, for extended operation according to the invention it is necessary to provide the first wash liquid inlet (and associated components) of the pressure diffuser of a material, such as stainless steel, having good corrosion resistant properties. For subsequent inlets and related components in the pressure diffuser conventional materials may be utilized. A washing liquid is applied to the other inlets in the pressure diffuser to effect washing of the pulp.
A pressure diffuser is particularly suited for the extended delignification according to the present invention. A pressure diffuser washes pulp at very high pressures and temperatures, and the diffusion action has been shown to be very effective. The apparatus according to the invention comprises a continuous digester with a pulp discharge at the bottom, and a multiple stage pressure diffuser having a pulp inlet, a pulp outlet, and a screened liquid outlet. As described above, the first stage is made of corrosion resistant material, and a treatment liquid inlet is provided to each stage. The apparatus further includes treatment means, such as a vacuum drum washer, having a pulp inlet, a pulp outlet, and a filtrate outlet; a source of white liquor; and liquid heating means. A first conduit transports pulp from the bottom of the digester to the pulp inlet of the pressure diffuser, while a second conduit transports pulp from the diffuser pulp outlet to the treatment means pulp inlet. A third conduit supplies a portion of the filtrate from the treatment means filtrate outlet, mixed with white liquor from the white liquor source, to the heating means. A fourth conduit supplies a heated mixture of white liquor and filtrate from the heating means to the treatment liquid inlet to the diffuser first stage, and a fifth conduit supplies wash liquid to the treatment liquid inlet of subsequent stages of the pressure diffuser, including at least some liquid from the filtrate outlet. Optionally, a sixth conduit may be provided for supplying screened liquid from the diffuser screened liquid outlet to the bottom of the digester for effecting a small amount of washing, with throttling valve means disposed in the sixth conduit for regulating the flow of screened liquid into the bottom of the digester. Any washing done in the digester is merely to effect removal of some dissolved lignin, but without reducing the pulp temperature to the extent that the desired K-number reduction is destroyed.
It is the primary object of the present invention to provide an effective method and apparatus for the extended kraft delignification of pulp. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.